Teresa Sharp, a homemaker and grandmother, has lived in Hamilton County, Ohio, for nearly 30 years. A former poll worker and a Democrat, she says she has voted in every election since she was 18.

"Voting to me is, like, sacred, like my children," she said. "It lets me at least have an opinion about how I want to live in America."

Sharp is keenly aware that her vote counts. Hamilton County, which includes Cincinnati, is hotly contested in a swing state that could decide this extraordinarily close presidential race. So naturally, Sharp was surprised when she received a letter in the mail that said, "You are hereby notified that your right to vote has been challenged by a qualified elector under RC 3503.243505.19."

Her husband, Herbert, her sons, Christopher and Herbert Jr., her daughters, Aseneth and Eleanor, and her elderly aunt, all residents at the same family home in Hamilton County, also received a similar letter.

"I thought to myself that there's somebody out here trying to scare people into not voting," she said.

The letter came from the county board of elections, and was prompted by an official challenge submitted by a member of The Ohio Voter Integrity Project, a local affiliate of a grassroots organization called True the Vote. The organization believes that voter fraud is a sweeping national epidemic and has enlisted and trained an army of citizen volunteers to challenge voters in the name of what they call "voter integrity."

Their promotional and recruitment videos talk about "willful, fraudulent behavior," and, "people voting who are not who they said they were." They said they address the important need to keep elections free and fair for all citizens.

The goals sound admirable, and even patriotic, but Sharp and other Democrats say True the Vote is less about voter integrity than voter suppression, and is specifically meant to intimidate minorities, low-income people and students who might vote for President Obama.

"I was like, 'Whoa, why are they targeting my family? What did we do?'" said Sharp.

While it is legal in 46 states for any citizen to challenge another citizen's right to vote, election officials said, it very rarely happens.

But this election cycle, the challenge against Sharp was among 1,077 citizen challenges received by the Hamilton County Board of Elections alone. The challenge against Sharp incorrectly cited her property as a vacant lot, but after a hearing Hamilton County election officials threw it out.

Attempts to reach Marlene Kocher, the member of the local Integrity Project who challenged Sharp, were not successful. Other True the Vote volunteers declined to speak with us on camera.

However, True the Vote founder Catherine Engelbrecht, who travels the country speaking about voter fraud, agreed to speak with "Nightline" in her first national television interview. Engelbrecht denied the allegations that her organization was attempting to intimidate voters from the polls.

"Our goal really is to encourage citizens to get involved in the process," she said. "It has been a continued shock and disappointment, frankly, to hear these allegations that continue to be leveled at us. It's unfortunate that there are those that have tried to take this and twist it into something that it's not."

Engelbrecht started True the Vote three years ago after serving as a poll watcher in Houston, where she said she witnessed things that disturbed her.

"All the way through to what can only be called voter fraud," she said. "We saw people who would come in with multiple registrations. We saw people who would come in and want to vote but their name had already been signed in the poll book. We recognized there's something not quite right."

Engelbrecht said that voter fraud can have a major impact on the outcome of an election.

"There have been many elections in the not-too-distant past that have been within such a tight margin that it really makes it very clear: Every vote counts," she said. "So you really begin to ask yourself, 'How much fraud's OK?'

"If there's a way to improve the process, and if there's a way for citizens to serve in the process and be a part of that contribution, then that's what we need to do," she added.

To stamp out voter fraud, True the Vote launched "integrity projects" all over the country by recruiting volunteers to start their own independent local chapters. True the Vote provides the groups with proprietary software to vet voter registrations, looking for inaccuracies that might be challengeable. Engelbrecht said the software compares the voter roll against the Social Security death index, property tax records and other public records.

"True the Vote is not turning in challenges," she said. "Citizen groups and private citizens across the country are using our software. ... The role, then, of a citizen volunteer is to look through. If you find anything, flag it and turn it over to the county officials."

Engelbrecht insisted that the organization is all about cleaning up voter rolls and denied that there is any political agenda behind the challenges.

"We do not attach party to anything that we do inside of the research," she said.

Teresa Sharp disagreed that challenging voters like her helped to keep elections fair.

While she agreed the challenge did not intimidate her, she expressed concern for others who might be challenged and not understand what it meant.

Election officials in Hamilton County agreed that the challenge against Sharp was ill-placed and that the challenger was clearly wrong in her research asserting Sharp's home was a vacant lot.

Alex Trianfilou, the Republican chair of the Hamilton County Board of Elections, said the local Integrity Project is doing a public service.

"They're reviewing the county and making sure our voter registration roles are accurate, and we think that's an important public service," he said.

Timothy Burke, the chairman of the Hamilton County Board of Elections and a Democrat, acknowledged that some of the challenges brought to the office correctly identified places where, say, a trailer park no longer existed. But others, he said, targeted college students who didn't list their dorm room or people who were mobile or lived in low-income areas -- and the majority of challenges were thrown out.

"Voter fraud has just not historically been a problem here in Hamilton County," Burke said. "I just think it's a smoke screen for their real effort, and that is to intimidate and prevent Democrats, and especially African-American Democrats, from voting."

Burke sees the challenges in context of what he said is a greater effort in Ohio to make voting more difficult, citing legislative moves to limit early voting days and pointing out billboards that went up in minority neighborhoods in several Ohio cities that said, "Voter Fraud is a Felony," with warnings of stiff fines and jail time.

"They're there for one purpose and that's to scare voters," he said. "There is a real concern that some voters have been confused, some voters have been intimidated, and the possibility that they might not show up because of it is a real shame."

Sharp also pointed out the billboards, and brought "Nightline" to one near her home.

"It makes me think back in the '20s and '30s and '40s when everything was segregated," she said, "you know, white people over there, and then black folks and everybody over there."

The billboard company in Cincinnati said the billboards were paid for by an anonymous family fund that wished to remain anonymous, and the billboards have since come down. "Nightline" found no evidence True the Vote was associated with the billboards.

Engelbrecht adamantly denied that True the Vote targets people based on race.

"Teresa Sharp has nothing to worry about because our citizens go into this race-blind, party-blind," she said. "This is literally nothing more than citizens doing what is legally allowed, what anyone can do in an effort to better our overall process and there is nothing more to it than that."

Engelbrecht conceded that the group's software program flags addresses with a high number of registered voters. When asked if the system was biased against people who live in multi-generational homes, she said, "That's the way we segment data just because it is an all-volunteer group that has only limited time."

In a video of an online training session for True the Vote volunteers, obtained by "Nightline" from an activist, an instructor said, "We have to exercise some discretion as we go through this too. You get the wrong person at the keyboard with this tool, if they were doing it for the wrong reasons they could get everyone in trouble. ... All they have to do is find a single judge that is sympathetic to their cause and it shuts us down in every state. So we have to be really careful about who we talk to and what we explain to people about how this thing works."

The instructor said the tool allows volunteers to vet across state lines, processing up to 3,000 voter registrations in 20 hours.

"If you have a friend that you know well, and you want to get him involved, I would just speak in generalities, don't go into real specifics," he said. "Because, like you, we would vet that person. They would sign a non-disclosure."

In response, Engelbrecht said that the remarks were taken out of context and were presented during the course of a longer conversation about the use of the group's publicly available research.

In a statement, she said, "True the Vote's research template was designed to identify potential inaccuracies in the voter rolls and support challenges or concerns by citizens so they may directly participate in the citizen challenge process. This important legal process was designed to protect voters from fraud and abuse, and relies on citizens to ensure the integrity of our voter rolls. Given the enormous value of this work, we, as an organization, take reasonable steps to help prevent the misuse of our data that supports the noble efforts of well-intended citizens who stand in the gap so every American vote is counted."

Engelbrecht insisted True the Vote is nonpartisan, and the volunteer training videos on its website do say that.

But there is evidence to suggest otherwise. Engelbrecht runs a Tea Party group in Houston and True the Vote recently donated $5,000 to a Republican organization.

Engelbrecht told "Nightline" the donation was a mistake, and said that her Tea Party group and True the Vote are two separate organizations. But True the Vote organizes national summits where members have made clear they want to see President Obama out of office.

"I'm not being over the top here: I fear the Obama gang is setting themselves up to steal the elections, if possible," one volunteer leader said.

But again, Engelbrecht denied that the group had a political agenda.

"Our agenda is to make sure that the election process is as free and as fair as it can be for all American voters," she said. "To the extent that the administration is standing in the way of that, I take exception."

True the Vote has said it is mobilizing one million poll watchers to go to voting places across the country. The problem, critics said, is that those watchers are mostly white and many of the polling places they target serve mostly black or minority voters.

In 2010, people in Texas complained that poll watchers who were affiliated with True the Vote were being overly aggressive and intimidating. According to Douglas Ray, the senior assistant county attorney for Harris County, Texas, the county where Engelbrecht lives, there were several complaints of True the Vote volunteers being disruptive to voters.

Ray said he went to the True the Vote offices and saw push pins on a map that he interpreted as the group's intention to target specific minority areas. This year, he said, his office has already received complaint calls about True the Vote volunteers at early voting locations.

The county attorney's office directed "Nightline" to an early voting location in Houston where there were Caucasian poll watchers in a predominantly African-American neighborhood, where citizens have already begun to complain.

Despite all the controversy the group has kicked up, study after study -- by the U.S. Department of Justice, investigative journalists and a bipartisan commission -- has found voter fraud to be virtually non-existent.

But Engelbrecht said her group's observations suggested there was "room for improvement." At last count, she said, her organization turned in 33 names to counties, and True the Vote was looking into close to 2,000 registrations that have almost identical matches. Although those might appear to be low numbers in context of the greater population, she said the 2000 presidential election was decided by 537 votes in Florida.

But Ray said voter fraud is not a current threat to democracy.

"I think that she is looking at things and perceiving a problem where a problem really doesn't exist," Ray said.

Whether Engelbrecht is dreaming it up or not, thousands of people agree with her and True the Vote volunteers will be showing up at polling places across the country this Election Day.

But Teresa Sharp is not deterred. In fact, she recently voted early in Hamilton County without incident.

"Too many people sacrificed their life for me to have that opportunity," she said.

"The remarks you refer to were taken out of context and are presented during the course of a longer conversation about the use of our publicly-available research. True the Vote's research template was designed to identify potential inaccuracies in the voter rolls and support challenges or concerns by citizens so they may directly participate in the citizen challenge process. This important legal process was designed to protect voters from fraud and abuse, and relies on citizens to ensure the integrity of our voter rolls. Given the enormous value of this work, we, as an organization, take reasonable steps to help prevent the misuse of our data that supports the noble efforts of well-intended citizens who stand in the gap so every American vote is counted.

We recognize that maintaining voter rolls can be a highly charged issue, full of fodder for false claims of impropriety -- and we want no part of it. The data, and our volunteers' work with it, should be treated with utmost integrity; at no time being used for partisan or personal purposes."